THz Beam Research Articles

Terahertz Wave Conversion From Linear to Circular Polarization by Graphene Metasurface Featuring Ultrawideband Tunability

This paper reports a detailed study of a graphene-based metasurface to function as tunable broadband linear to circular polarization converter in the terahertz gap. This device provides a wide polarization conversion band of 2.25 THz (3.75 THz to 6 THz) with an enhanced fractional 3-dB axial ratio bandwidth (FARBW) of 46.15%. The proposed structure consists of a simple tri-layer configuration where the meta-atom comprises a patterned graphene layer on a gold-backed silicon dioxide substrate. The proposed reflective-type linear to circular polarization converter (RLCPC) exhibits both right-handed and left-handed circular orientations of the reflected wave in two halves of the operating spectrum, further validated in terms of polarization ellipses developed by an inhouse code. The lack of experimental setup at the above said frequencies available to the authors in their country has been paid off by verifying the response using a detailed circuit model. The RLCPC maintains a thickness of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_o</i> /7.69 with a lattice size of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_o</i> /8.33. The structure also promises a stable wideband circular polarization conversion up to 20° and a dual-band circular polarization conversion up to 40° incident angle of the electromagnetic wave. This device can be useful for THz beam scanning antennas, RCS reduction and THz antennas for space and military radar applications.

Electromagnetic multi-beam steering of matrix pattern-encoded metasurfaces

An electromagnetically encoded metasurface is a synthetic surface based on subwavelength unit particles, which can freely control the amplitude, phase and polarization direction of electromagnetic waves. Using digital states to characterize electromagnetic parameters directly links digital technology at the information level with metasurface technology at the physical level. This paper proposes a matrix encoding mode to realize the flexible control of the number of scattering beams and the scattering angle of the encoding metasurface. In order to reduce the ohmic loss of the metal structure metasurface, we propose to use the all-dielectric material cylindrical structure to construct the coding unit. In view of the encoding of metasurface sequences in one-dimensional direction, the scattering angle of THz beam can only be controlled in a single direction, but not the scattering azimuth in three-dimensional (3D) space. We use different matrix encoding modes to achieve multi-beam and multi-angle control of terahertz beams in 3D space. Based on the cross-coding mode, the multi-beam separation of the beam scattering main lobe is obtained, and the azimuth angle of each separated main lobe beam is flexibly controlled.

Photonic-assisted 2-D terahertz beam steering enabled by a LWA array monolithically integrated with a BFN.

A novel photonic-assisted 2-D Terahertz beam steering chip using only two tuning elements is presented. The chip is based on an array of three leaky wave antennas (LWAs) with a monolithically integrated beamforming network (BFN) on a 50 µm-thick indium phosphide substrate. The THz beam angle in elevation (E-plane) is controlled via optical frequency tuning using a tunable dual-wavelength laser. An optical delay line is used for azimuth (H-plane) beam control. The simulated beam scanning range is 92° in elevation for a frequency sweep from 0.23 THz to 0.33 THz and 69.18° in azimuth for a time delay of 3.6 ps. For the frequency range from 0.26 THz to 0.32 THz, it is confirmed experimentally that the THz beam scans from -12° to +33°, which is in good agreement with the numerical simulations. The beam direction in azimuth scans with a total angle of 39° when applying a delay difference of 1.68 ps. A good agreement is found between theoretically predicted and experimentally determined THz beam angles with a maximum angle deviation below 5°. The experimental scanning angles are limited due to the mechanical constraints of the on-wafer probes, the on-chip integrated transition and the bandwidth of the THz receiver LNA. The mechanical limitation will be overcome when using a packaged chip.

Generation and beam shaping of THz radiation in an actively modulated nonlinear crystal

Broadband THz pulses can be generated by optical rectification of ultrashort visible or near-infrared laser pulses in a second-order nonlinear optical crystal. Here, we propose and demonstrate theoretically that THz beam-shaping and steering can be achieved by properly modulating the second-order susceptibility, χ(2) of a nonlinear crystal in its transverse plane. Different duty cycles of the periodic χ(2) -modulations provide the main control for the angular steering of the generated THz beam away from the direction of the undeflected beam from a uniform crystal. Zinc telluride crystal of thickness 1 mm is considered for the demonstration of the scheme in which optical rectification of 100 fs excitation pulses, centered at 800 nm, resulted in a THz beam of ∼0–3.5 THz bandwidth pulses. Angular steering of ∼7.4 degrees could be achieved in comparison to that of the undeflected THz beam case using a ZnTe crystal of the same thickness but without any periodic modulations of the nonlinear susceptibility. The THz power distribution around the central lobe of the deviated THz beam depends on the duty cycle of the modulation. Another consequence of the periodically χ(2)-structured nonlinear crystal is that the steered THz beam is focused at a particular point in the observation plane, hence, multiple splitting of the THz beam can be achieved by varying the number of modulation periods in the crystal.

Programmable VO2 metasurface for terahertz wave beam steering

SummaryProgrammable vanadium dioxide (VO2) metasurface is proposed at THz frequencies. The insulating and metallic states of VO2 can be switched via external electrical stimulation, resulting in the dynamical modulation of electromagnetic response. The voltages of different columns of the metasurface can be controlled by the field-programmable gate array, and thus the phase gradients are realized for THz beam steering. In 1-bit coding, we design periodic and nonperiodic 24 × 24 coding sequences, and achieve wide-angle beam scanning with the deflection angles from −60° to +60°. In 2-bit coding, we use two different meta-atoms to design 18 × 18 coding sequences. Compared with 1-bit coding, 2-bit coding has more degree of freedom to control the optical phase, and 3 dB diffraction efficiency is improved by generating a single deflection angle. The proposed programmable metasurfaces provide a promising platform for manipulating electromagnetic wave in 6G wireless communication.

THz Filters Made by Laser Ablation of Stainless Steel and Kapton Film.

THz band-pass filters were fabricated by femtosecond-laser ablation of 25-m-thick micro-foils of stainless steel and Kapton film, which were subsequently metal coated with a ∼70 nm film, closely matching the skin depth at the used THz spectral window. Their spectral performance was tested in transmission and reflection modes at the Australian Synchrotron’s THz beamline. A 25-m-thick Kapton film performed as a Fabry–Pérot etalon with a free spectral range () of 119 cm, high finesse , and was tuneable over ∼m (at ∼5 THz band) with tilt. The structure of the THz beam focal region as extracted by the first mirror (slit) showed a complex dependence of polarisation, wavelength and position across the beam. This is important for polarisation-sensitive measurements (in both transmission and reflection) and requires normalisation at each orientation of linear polarisation.

Terahertz imaging using optically controlled Fourier-basis structured illumination.

We demonstrate that a new type of structured-illumination imaging may be migrated from the optical to the terahertz domain. This Fourier-basis technique involves illuminating a target with rapidly moving sinusoidal fringes of controllable spatial frequency and orientation, while measuring the scattered radiation on a single fast detector. This initial proof-of-concept demonstration is purely one-dimensional since the fringe orientation is fixed, but the technique is readily extensible to two dimensions. The fringes are first generated in the near-infrared (808 nm) by passing a high-power laser beam through an acousto-optic Bragg cell driven by a superposition of two RF signals slightly offset in frequency, blocking the undeflected beam, and refocusing the two diffracted beams onto a metal-backed semiconductor wafer. The laser can be amplitude modulated to slow down the moving fringes to accommodate the semiconductor's temporal response. The semiconductor acts as an optically addressed spatiotemporal modulator for a THz beam illuminating the same area. The periodic optical fringes effectively transform the semiconductor into a reflective THz diffraction grating with a programmable period. The diffracted THz radiation is then imaged onto the remote target plane, where the diffraction orders interfere pairwise to create traveling THz fringes. Scattered radiation from the target is collected by a simple receiver operating in "light bucket" mode, which produces an output signal consisting of a superposition of sinusoidal tones, one for each spatial Fourier component of the target. We present measurements of the THz fringe projector's performance and compare with a model of the semiconductor modulator's operation. Finally, we present Fourier-reconstructed images of pairs of point targets as an initial demonstration of THz Fourier-basis agile structured illumination sensing imaging.

THz beam shaping based on diffractive transformation for forming patterned simulation lightfields and wavefronts

In this paper, the traditional Gerchberg-Saxton (GS) algorithm is used to achieve a key phase recovery in the THz band via common Fourier transformation to realize desired beam manipulation. The main technological process includes a single mask UV-photolithography and a dual-step KOH wet etching, thus enabling a kind of THz diffractive optical element (THz-DOE) with the needed phase-step arrangement. The experimental results about THz beam shaping for generating patterns and wavefronts based on the THz-DOEs are given to demonstrate the feasibility of realizing complex lightfield generation and modulation for THz imaging applications. The developed THz-DOEs exhibit a significant THz amplitude modulation for constructing some particular patterns selected. The non-ideal surface roughness and phase-step configuration of the silicon-based DOEs for both the visible and infrared wavelength ranges is perfectly acceptable in the THz band due to its long wavelength nature. A better imaging exhibition based on the beam modulation of the developed THz-DOEs can be expected since the phase configuration accuracy can be further improved according to the technological and micro-structural parameter optimization.

Experimental Demonstration of Sub-THz Wireless Communications Using Multiplexing of Laguerre-Gaussian Beams When Varying two Different Modal Indices

The ability to multiplex multiple structured beams for sub-terahertz (sub-THz) wireless communications has been recently explored. The spatial orthogonality of structured beams enables mode-division multiplexing (MDM). In an MDM system, multiple data-carrying beams are transmitted and received simultaneously through a single aperture pair with low inherent crosstalk. This can increase data capacity and spectral efficiency of the system. Here, we experimentally demonstrate multiplexing orthogonal sub-THz Laguerre-Gaussian (<inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{\ell,{{\bf p}}}}$</tex-math></inline-formula>) beams when varying two different modal indices (i.e., both <inline-formula><tex-math notation="LaTeX">$\ell $</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">${{\bf p}}$</tex-math></inline-formula>); this contrasts with prior reports when varying only one index (i.e., <inline-formula><tex-math notation="LaTeX">$\ell $</tex-math></inline-formula>) and could potentially provide a larger set of channels and beams in an MDM system. In our demonstration, specially designed phase patterns are used: (1) at the transmitter, to convert two THz Gaussian beams to two different LG beams, each carrying an independent data channel; and (2) at the receiver, to separate and convert the LG beams back to Gaussian-like THz beams. An 8-Gbit&#x002F;s quadrature-phase-shift-keying (QPSK) link containing two multiplexed LG modes over 40 cm is experimentally demonstrated. For the above link, three different LG modal sets are chosen (i.e., {<inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{ - 2,0}}$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{1,1}}$</tex-math></inline-formula>} or {<inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{3,0}}$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{0,1}}$</tex-math></inline-formula>} or {<inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{2,1}}$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{0,1}}$</tex-math></inline-formula>}). All channels are recovered with bit errorrates (BERs) below the 7&#x0025; forward error correction (FEC) limit. The experimental results indicate that: (a) higher-order LG modes experience more divergence, larger size, and lower conversion efficiency, contributing to higher power loss; and (b) the modal coupling and crosstalk for different LG modes is &lt;&#x2212;12 dB, which could be due to the transmitter&#x002F;receiver misalignments as well as beam truncation by the limited-sized receiver aperture.

Accurate THz ellipsometry using calibration in time domain.

We report on the realisation of a customized THz time domain spectroscopic ellipsometer (THz-TDSE) based on fiber-coupled photoconductive antennas, operating in a wide range of incident angles and allowing also standard transmission spectroscopy without any optical realignment. To ensure accurate parameter extraction for a broad range of materials, we developed a fast and effective algorithm-assisted method to calibrate the setup and compensate for the nonideality in the response of the THz system. The procedure allows to minimise errors induced by imperfect response of the antennas and polarizers, imprecise setting of the impinging and receiving angles in the goniometric mechanical arms, and unavoidable mismatches in the THz beam optics. Differently from other calibration methods applied in the literature, our approach compares in time domain the ellipsometric derived electric field s- and p-polarised components at a given angle of incidence with the reconstructed ones, attained by using the complex dielectric function of a known sample. The calibrated response is determined with high precision by setting the system in transmission mode. In order to validate the technique, ellipsometric measurements have been carried out at various angle of incidences on a number of materials both in solid and liquid form, and their data compared with what obtained by conventional THz spectroscopy. Results show that THz-TDSE accompanied with an accurate calibration procedure is an effective technique for material characterization, especially in case of samples with a high absorption rate that are not easily investigated through transmission measurements.

Bandwidth of a 4.7 THz beam multiplexer based on Fourier grating

We present an analysis of the bandwidth of an asymmetric 8-beam Fourier grating as the beam multiplexer for a 4.7 THz local oscillator used in a heterodyne receiver. We take the grating designed for NASA GUSTO balloon observatory as an example to address the bandwidth question although it does not need to operate over a wide frequency range. By illuminating the grating at different frequencies from 4.445 to 5.045 THz, we simulated the changes of its performance in three aspects using COMSOL Multiphysics: diffraction efficiency, power uniformity, and the angular distribution of the output beams. These parameters can affect the coupling efficiency between the output beams of the grating and the beams of a mixer array. The bandwidth of the grating is found to be 230 GHz, corresponding to 4.9% of the operating frequency, which is sufficient for many applications.

THz-photonics transceivers by all-dielectric phonon-polariton nonlinear nanoantennas

The THz spectrum (spanning from 0.3 to 30 THz) offers the potential of a plethora of applications, ranging from the imaging through non transparent media to wireless-over-fiber communications and THz-photonics. The latter framework would greatly benefit from the development of optical-to-THz wavelength converters. Exploiting Difference Frequency Generation in a nonlinear all dielectric nanoantenna, we propose a compact solution to this problem. By means of a near-infrared pump beam (at omega _1), the information signal in the optical domain (at omega _2) is converted to the THz band (at omega _3=omega _2-omega _1). The approach is completely transparent with respect to the modulation format, and can be easily integrated in a metasurface platform for simultaneous frequency and spatial moulding of THz beams.

Spintronic emitters for super-resolution in THz-spectral imaging

We investigate local THz field generation using spintronic THz emitters to enhance the resolution for micrometer-sized imaging. Far-field imaging with wavelengths above 100 μm limits the resolution to this order of magnitude. By using optical laser pulses as a pump, THz field generation can be confined to the area of laser beam focusing. The divergence of the generated THz beam due to laser beam focusing requires the imaged object to be close to the generation spot at a distance below the THz field wavelength. We generate THz-radiation by fs-laser pulses in CoFeB/Pt heterostructures, based on spin currents, and detect them by commercial low-temperature grown-GaAs (LT-GaAs) Auston switches. The spatial resolution of THz radiation is determined by applying a 2D scanning technique with motorized stages allowing step sizes in the sub-micrometer range. Within the near-field limit, we achieve spatial resolution in the dimensions of the laser spot size on the micrometer scale. For this purpose, a gold test pattern is evaporated on the spintronic emitter separated by a 300 nm SiO2 spacer layer. Moving these structures with respect to the femtosecond laser spot, which generates THz radiation, allows for resolution determination. The knife-edge method yields a full-width half-maximum beam diameter of 4.9±0.4 μm at 1 THz. The possibility to deposit spintronic emitter heterostructures on simple glass substrates makes them attractive candidates for near-field imaging in many imaging applications.

Dynamic full-field refractive index distribution measurements using total internal reflection terahertz digital holography

Massive usage scenarios prompt the prosperity of terahertz refractive index (THz RI) measurement methods. However, they are very difficult in measuring the full-field dynamical RI distributions of either solid samples without a priori thickness or liquid samples. In this study, we propose total internal reflection THz digital holography and apply it for measuring RI distributions for both solid and liquid samples dynamically. An RI measurement model is established based on an attenuated total reflection prism with a pitching angle. The pitching angle and the field of view can be numerically calculated from the spectrogram of the off-axis Fresnel hologram, which solves the adjustment of the visually opaque prism irradiated by the invisible THz beam. Full-field RI distributions of the droplets of solid-state soy wax and distilled water are obtained and compared with THz time-domain spectroscopy. The evaporation of an ethanol solution droplet is recorded, and the variation of the RI distribution at the sample–prism interface is quantitatively visualized with a temporal resolution of 10 Hz. The proposed method greatly expands the sample range for THz RI measurements and provides unprecedented insight into investigating spontaneous and dynamic THz phenomena.

Optimum Design of Electron Gun for 0.22-THz Traveling Wave Tubes

To improve the performance of a pencil beam electron gun for terahertz traveling wave tubes (TWT), a support vector regression (SVR) method combined with a goal attainment algorithm, which is used to optimize its structural parameters considering the periodic permanent magnetic (PPM) focusing system, is proposed in this article. Moreover, a parameter correction method to reduce the impact of the thermal expansion on the electron gun is proposed. An electron gun for 0.22-THz TWT gun is developed, in which a 30-mA/23.8-kV electron beam emitted by a curved barium–tungsten cathode with a beam loading of 4 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is achieved with the optimized parameters. To the high current density for THz beam–wave interaction, the area compression ratio of electron beam is used in beam tunnel and reaches 144. The experimental results are very similar to the simulation results, which indicate that the methods would be beneficial to the development of TWT.

Observation of quasi-monochromatic resonant Cherenkov diffraction radiation

The first observation and investigation of a new mechanism of resonant Cherenkov diffraction radiation appearing when relativistic 6 MeV electrons move alongside a periodically shaped Teflon target have been presented and analysed. Numerical simulations performed using computer code KARAT are in good agreement with the experimental results. This new mechanism is a promising technique for generation and monochromatisation of THz and sub-THz radiation beams that could be integrated into any short bunch linear accelerator facility including 4th generation light sources providing new opportunities for user community.

Polarization-independent all-dielectric guided-mode resonance filter according to binary grating and slab waveguide dimensions.

All-dielectric binary gratings, with and without slab waveguides, are designed to generate polarization-independent guided-mode resonance filters (GMRFs) operating in the THz frequency region using the rigorous coupled-wave analysis (RCWA) method. The filling factor and thickness of the grating were adjusted to have equal resonance frequencies of transverse electric (TE)- and transverse magnetic (TM)-polarized THz beams. The single polarization-independent resonance for a binary grating without a slab waveguide was obtained at 0.459 THz with full width at half maximum (FWHM) values of 8.3 and 8.5 GHz for the TE and TM modes, respectively. Moreover, double-layered polarization-independent resonances for binary gratings with slab waveguides were obtained at 0.369 and 0.442 THz with very high Q-factors of up to 284. This is the first study to propose a polarization-independent GMRF with two resonant frequencies.

Mobile THz communications using photonic assisted beam steering leaky-wave antennas.

THz communications is envisaged for wide bandwidth mobile communications eventually reaching data capacities exceeding 100 Gbit/s. The technology enabling compact chip-integrated transceivers with highly directive, steerable antennas is the key challenge at THz frequencies to overcome the very high free-space path losses and to support user mobility. In this article, we report on mobile and multi-user THz communications using a photonic THz transmitter chip featuring 1D beam steering for the first time. In the proposed approach, 1D THz beam steering is achieved by using a photodiode excited leaky-wave antenna (LWA) in the transmitter chip. The on-chip LWA allows to steer the directive THz beam from 6° to 39° within the upper WR3-band (0.28-0.33 THz). The antenna's directivity is 14 dBi which is further increased to 23 dBi using an additional hemicylindrical Teflon lens. The 3-dB beam width and coherence bandwidth of the fabricated THz transmitter chips with lens are 9° and 12 GHz, respectively. The proposed approach allows steering the THz beam via the beat frequency of an optical heterodyne system at a speed up to 28°/s. Without using a THz amplifier in the transmitter chip, a data rate of 24 Gbit/s is achieved for a single user for all beam directions and at short wireless distances up to 6 cm. The wireless distance is successfully increased to 32 cm for a lower data rate of 4 Gbit/s, still without using a transmitter amplifier. Also, multi-user THz communications and the overall capacity of the developed THz transmitter chip is studied revealing that up to 12 users could be supported together with a total wireless data capacity of 48 Gbit/s. Fully integrated 2D transmitter chips are expected to reach wireless distances of several meters without additional amplifiers.

Analysis of THz generation using the tilted-pulse-front geometry in the limit of small pulse energies and beam sizes.

Optical rectification in lithium niobate using the tilted-pulse-front geometry is one of the most commonly used techniques for efficient generation of energetic single-cycle THz pulses and the details of this generation scheme are well understood for high pulse energy driving lasers, such as mJ-class, kHz-repetition rate Ti:Sa amplifier systems. However, as modern Yb-based laser systems with ever increasing repetition rate become available, other excitation regimes become relevant. In particular, the use of more moderate pulse energies (in the few µJ to multi-10 µJ regime), available nowadays by laser systems with MHz repetition rates, have never been thoroughly explored. As increasing the repetition rate of THz sources for spectroscopy becomes more relevant in the community, we present a thorough numerical analysis of this regime using a 2+1-D numerical model. Our work allows us to confirm experimental trends observed in this unusual excitation regime and shows that the conversion efficiency is naturally limited by the small pump beam sizes as a consequence of spatial walk-off between the pump and THz beams. Based on our findings, we discuss strategies to overcome the current limitations, which will pave the way for powerful THz sources approaching the watt level with multi-MHz repetition rates.

Numerical Study of the Coupling of Sub-Terahertz Radiation to n-Channel Strained-Silicon MODFETs.

This paper reports on a study of the response of a T-gate strained-Si MODFETs (modulation-doped field-effect transistor) under continuous-wave sub-THz excitation. The sub-THz response was measured using a two-tones solid-state source at 0.15 and 0.30 THz. The device response in the photovoltaic mode was non-resonant, in agreement with the Dyakonov and Shur model for plasma waves detectors. The maximum of the photoresponse was clearly higher under THz illumination at 0.15 THz than at 0.3 THz. A numerical study was conducted using three-dimensional (3D) electromagnetic simulations to delve into the coupling of THz radiation to the channel of the transistor. 3D simulations solving the Maxwell equations using a time-domain solver were performed. Simulations considering the full transistor structure, but without taking into account the bonding wires used to contact the transistor pads in experiments, showed an irrelevant role of the gate length in the coupling of the radiation to the device channel. Simulations, in contradiction with measurements, pointed to a better response at 0.3 THz than under 0.15 THz excitation in terms of the normalized electric field inside the channel. When including four 0.25 mm long bonding wires connected to the contact pads on the transistor, the normalized internal electric field induced along the transistor channel by the 0.15 THz beam was increased in 25 dB, revealing, therefore, the important role played by the bonding wires at this frequency. As a result, the more intense response of the transistor at 0.15 THz than at 0.3 THz experimentally found, must be attributed to the bonding wires.